System for contactless testing of printed circuit boards

ABSTRACT

A system for testing for opens in and shorts between conductor traces on a printed circuit board is provided. The system includes an electron gun assembly for generating an electron beam and an electron optics assembly for directing the electron beam to the traces on the surface of the board, the traces having a reference potential. A grid located proximate to and substantially parallel with the surface of the board is placed at a first potential before the electron beam is directed to a first point on a trace to charge the trace to a first potential. The grid is then placed at a second potential, the second potential being between the reference potential and the first potential, before the electron beam is directed to a second point on the trace to cause emission of secondary electrons. The secondary electrons that reach the grid are collected by the grid, and signal processing electronics and a CPU are used to determine whether or not an open or short exists depending on whether secondary electrons are collected by the grid as the electron beam is directed to various locations on the traces. The position and intensity of the electron beam are controlled by a raster/vector generator. In another aspect, the present invention provides a system having two electron gun assemblies and two grids, one of each placed on each side of a board so that traces on both sides of the board, or traces passing through the board, can be tested simultaneously.

FIELD OF THE INVENTION

The present invention relates to a system for testing the networks ofprinted circuit boards without any physical contact therewith, and moreparticularly to a system and method of using an electron beam to testfor shorts between and opens in conductor traces of a printed circuitboard.

BACKGROUND OF THE INVENTION

It is desirable in the manufacture of printed circuit boards to test fordefects at an early stage of fabrication in order to minimize the costsof repairing such defects and to maximize the yield of operable devices.The traditional approach to testing such circuitry has been to usemechanical devices, such as pins or other mechanical probes, to contactpoints on the circuitry to be tested and to run a current therethrough.However, the small size and high density of modern printed circuitboards makes the use of such mechanical testing devices unfeasible. Thenetworks of modern printed circuit boards are becoming so small and socompactly placed on the boards that using a mechanical probe isdifficult, if not impossible. Moreover, as the networks become smaller,the risks associated with damaging the networks using mechanical probesincreases. For these reasons, systems using electron beams to providecontactless testing have been developed. However, although such systemsexist, these known systems suffer from a number of disadvantages.

U.S. Pat. No. 4,843,330 to Golladay et al. discloses an electron beamcontactiess testing system which includes a conductive grid 48 placedabove the surface of the specimen 36 to be tested . The 48 grid isnegatively biased while the networks to be tested are charged in orderto repel secondary electrons back toward the specimen and enhancecharging of the conductive material. The bias is removed, or a positivebias is applied, during scanning of the specimen by a beam 12 so thatsecondary electrons can be collected by a detector. The testing systemalso includes a table 32 selectively movable in the X-Y directionsperpendicular to the axis of beam 12 to position specimen 36 within thebeam deflection field.

U.S. Pat. No. 5,602,489 to El-Kareh et al. discloses a method fortesting the interconnect networks of a multichip module for opens andshorts. An electron beam 570 lands on a pad of an interconnect networklocated on a substrate 500. The electron beam 570 is used to interrogatethe pad. An extract grid 550 located above the substrate is maintainedat a positive potential. While the electron beam 570 interrogates thepad, the pad emits secondary electrons until such a point that the padreaches a positive potential near that of the positive potential of theextract grid 550. The extract grid is then switched to a negativepotential. The pad, still being interrogated by the electron beam 570,then collects secondary electrons until such a point that the padreaches a negative potential near that of the negative potential of theextract grid 550. The test time, the length of time it takes for the padto change from the positive potential to the negative potential, ismeasured and compared to a reference value. From this comparison it canbe determined whether the interconnect network is defect-free, open, orshorted. The increase or decrease in emitted secondary electrons areevaluated by an electron detector 560, and the substrate 500 is movedrelative to the electron beam 570 by an x-y stage 540.

U.S. Pat. No. 4,169,244 to Plows discloses a system for testingelectronic networks. The system includes an electron gun 2 forproduction of an electron probe 1, a scanning control for probe 1, and aholder 16 capable of supporting a specimen 25 such that the probe 1 canimpinge substantially normally on the specimen 25. Deflection coils 6are used to direct electron probe 1 to varying locations on the specimen25.

All of these prior art patents disclose devices which use a focusedelectron beam, such as that generated by a scanning electron microscope.Such a beam is capable of charging/reading a small area, for example anarea of 1 inch by 1 inch. When a larger board, or a panel of smallboards, is to be tested it is therefore necessary to move the board (asdoes the X-Y table of Golladay et al. and the x-y stage of El-Kareh etal.). Moving the board, however, can greatly increase the time necessaryfor testing, particularly when numerous points on the board must betested.

Moreover, all prior are references disclose systems which can performonly top-to-top testing of board networks. If the network passes throughthe board, the network cannot be charged at a point on one side of theboard and then tested at a point on the other side of the board. This isa serious disadvantage, as many modern circuit boards have networkswhich pass therethrough.

Furthermore, the systems disclosed in all prior art patents require thatsecondary electrons emitted by the networks being tested be collected bya discrete electron detector. Such systems are prone to error, however,as emitted electrons may not necessarily be deflected toward thedetector, and may therefore not be detected.

A further disadvantage with respect to El-Kareh et al. is that the testtime, that is, the length of time it takes for the pad to change fromthe positive potential to the negative potential, must be measured andcompared to a reference value for every pad tested. Depending on thematerials used and the size of the pads, this test time may be great,thereby greatly increasing the time necessary for testing a board.

What is desired, therefore, is a system for testing printed circuitboards which tests the networks of printed circuit boards without anyphysical contact therewith, which quickly tests numerous points on theboards, which can test large boards or panels of small boards withoutrequiring movement of the boards, which tests networks passing throughthe boards, and which reliably detects the presence of emitted secondaryelectrons.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asystem for testing printed circuit boards which employs electron beamsto test the networks of printed circuit boards without any physicalcontact therewith.

Another object of the present invention is to provide a system fortesting printed circuit boards which quickly tests numerous points onthe boards.

A further object of the present invention is to provide a system fortesting printed circuit boards which employs an electron beam such thatlarge boards and panels of small boards can be tested without requiringmovement of the boards.

Still another object of the present invention is to provide a system fortesting printed circuit boards which tests networks passing through theboards.

Still a further object of the present invention is to provide a systemfor testing printed circuit boards which tests both sides of printedcircuit boards using an electron beam on each side.

Yet a further object of the present invention is to provide a system fortesting printed circuit boards which reliably detects the presence ofemitted secondary electrons.

These and other objects of the present invention are achieved byprovision of a system for testing for opens in and shorts betweenconductor traces on a surface of a printed circuit board. The systemincludes an electron gun assembly for generating an electron beam and anelectron optics assembly for directing the electron beam to theconductor traces on the surface of the printed circuit board, whichconductor traces are at a reference potential. A grid located proximateto and substantially parallel with the surface of the printed circuitboard is placed at a first potential before the electron beam isdirected to a first point on a conductor trace to charge the conductortrace to a first potential. The grid is then placed at a secondpotential, the second potential being between the reference voltage andthe first potential, before the electron beam is directed to a secondpoint on the conductor trace to cause emission of secondary electrons.Any secondary electrons that reach the grid are collected by the grid,and a CPU is used to determine whether or not an open or short existsdepending on whether secondary electrons are collected by the grid asthe electron beam is directed to various locations on the conductortraces. The position and intensity of the electron beam are controlledby a raster/vector generator.

The electron optics assembly includes a focus coil having static anddynamic windings to focus the electron beam anywhere across the surfaceof the printed circuit board, at least one beam alignment yoke toposition the electron beam precisely along the magnetic axis of thefocus coil, a deflection yoke to produce magnetic fields that preciselyposition the electron beam anywhere across the surface of the printedcircuit board, and an astigmatism corrector to compensate for anyresidual astigmatism caused by the focus coil and for any deflectionastigmatism caused by the deflection yoke. The electron optics assemblyalso includes a static focus control for controlling the static windingof the focus coil and a dynamic focus driver for controlling the dynamicwindings of the focus coil to adjust the focus of the electron beam forchanges in focal length as the electron beam is deflected over thesurface of the printed circuit board. A dynamic astigmatism correctiongenerator is provided for controlling the astigmatism corrector tocorrect for spot distortions as the electron beam is deflected away froma center of the printed circuit board, and a geometric correctiongenerator is provided for controlling the deflection yoke to correct forpositional distortions as the electron beam is deflected away from acenter of the printed circuit board.

Preferably, the electron gun assembly is surrounded by a gun chamber,the electron optics assembly is surrounded by an electron optics chamberand the printed circuit board is surrounded by a test chamber, and thesystem includes a vacuum system. The vacuum system creates a vacuum inthe test chamber, a vacuum in the electron optics chamber greater thanthe vacuum in the test chamber, and a vacuum in the gun chamber greaterthan the vacuum in the electron optics chamber.

Also preferably, the system utilizes a spot analyzer for adjustingfocus, astigmatism and geometry of the electron beam, and for measuringthe spot size of the electron beam. The spot analyzer includes a targetplate having a plurality of polygonal apertures on the face. A pluralityof sensors are positioned inside the target plate under the plurality ofpolygonal apertures. The signals from the sensors are indicative of thequality of an electron beam striking the polygonal aperture above eachsensor. Beam control signals are provided by the raster/vector generatorfor directing the beam to strike the apertures above the sensors. Signalprocessing electronics are also provided for amplifying, filtering, andperforming analog-to-digital conversion of the signals generated by thesensors and for generating processed signals, which can be used bycomputational software executing the CPU to adjust the electron opticsassembly.

In another aspect, the target plate can contain a plurality of polygonalapertures on two opposing faces with the plurality of sensors designedto receive signals from the plurality of sensors on both faces. Such aconfiguration will permit aligning of two sets of electron optics in atwo-sided, dual beam testing system.

Most preferably, each surface of the printed circuit board includes atleast two fiducial marks thereon. Software executing on the CPU alignsthe deflection axes of each of the electron optics assemblies withrespect to each surface of the printed circuit board based uponsecondary electron signals generated by the fiducial mark(s).

In another aspect, the present invention provides a system having twoelectron gun assemblies, two electron optics assemblies and two grids,one of each placed on each side of a printed circuit board having twofaces so that traces on both sides of the board, or traces passingthrough the board, can be tested simultaneously.

The invention and its particular features and advantages will becomemore apparent from the following detailed description considered withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side isometric view of a portion of a system for testing foropens in and shorts between conductor traces on a surface of a printedcircuit board in accordance with the present invention;

FIG. 2 is a schematic view of an electron gun assembly which may be usedin the system of FIG. 1;

FIG. 3 is a schematic view of the system of FIG. 1;

FIG. 4 is a schematic view of a vacuum system which may be used in thesystem of FIG. 1;

FIG. 5 is a schematic view of a dual beam system that can test bothsides of a printed circuit board simultaneously.

FIG. 6 is a block diagram of a spot analyzer which may be used with thesystem of FIG. 1;

FIG. 7 is a side isometric view of a portion of the spot analyzer ofFIG. 6;

FIGS. 8 and 9 are end plan views illustrating the methodology used bythe system of FIG. 1 to test printed circuit boards;

FIGS. 10-13 are top plan views of simple printed circuit boards whichmay be tested using the system of FIG. 1;

FIGS. 14 and 15 are end plan views illustrating the methodology used bythe system of FIG. 1 to test printed circuit boards; and

FIG. 16 is a side isometric view, partially cut away, illustrating aprinted circuit board being tested using the system of FIG. 1;

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIGS. 1-5, a system 10 for contactless testing ofprinted circuit boards in accordance with the present invention isshown. System 10 includes an electron gun assembly 12, an electronoptics assembly 14, an electron optics chamber 16, and a test chamber18. It should be noted that system 10 preferably includes two electrongun assemblies 12, two electron optics assemblies 14 and two electronoptics chambers 16, as shown in FIG. 5. However, only one of each isshown in FIGS. 1 and 3 for the sake of simplicity. The description belowrefers to such a dual-beam system, although a single-beam system is alsocontemplated. System 10 also includes a vacuum system 20 (shown best inFIG. 4) located within a housing 22. A central processing unit (CPU) 24is also provided to control various operations of system 10 as discussedmore fully below.

Referring now to FIGS. 2 and 3, electron gun assemblies 12 are used togenerate electron beams (indicated by arrows 26), and generally includethree components: a filament 28, a gun grid 30 and an anode 32 locatedwithin a gun chamber 34. The filament 28, or cathode, is heated bypassing a DC current, supplied by a DC power supply (discussed below),through it. This current frees some of the electrons on the surface offilament 28. The free electrons are accelerated toward anode 32 by anelectrostatic field produced by a difference in potential betweenfilament 28 and the anode 32. The gun grid 30, which is placed betweenfilament 28 and anode 32, but much closer to filament 28, is connectedto a slightly more negative potential than filament 28. A hole 36 in thecenter of gun grid 30 permits electrons to pass from filament 28 toanode 32. The electrostatic field produced by the difference inpotential between gun grid 30 and filament 28 opposes the field providedbetween filament 28 and anode 32. This field repels some of theelectrons emitted from filament 28 and prevents them from passingthrough hole 36 in gun grid 30 toward anode 32. By varying the voltageapplied to gun grid 30, the electron stream from filament 28 to anode 32can be increased, decreased or turned off completely.

The electrons that pass through hole 36 in gun grid 30 strike anode 32.An extremely small aperture 38 in anode 32 allows some of the electronsto pass therethrough, forming electron beam 26. The electrons strikinganode 32, but not passing through aperture 38, are returned to groundpotential through an anode termination assembly 40 since anode 32 isisolated from ground inside gun chamber 34. A coupler 42 receives anintensity signal from a raster/vector generator 43 and couples it to gungrid 30, which is referenced at a reference voltage, in order to controlgun grid 30 to modulated the intensity of electron beam 26.Raster/vector generator 43, with its software, controls the position andintensity of electron beams 26. In addition to accurately positioningelectron beams 26, it can scan electron beams 26 in selected rasterformats as well as in selected vector formats. Raster/vector generator43 can preferably provide 65K by 65K addressability, up to 4096 beamintensity levels, and size selectable rasters and subrasters. Allcapabilities of raster/vector generator 43 are under software control,which may or may not be executing on CPU 24.

The power for electron gun assemblies 12 is provided by four powersupplies which flow through a distribution module 46. Reference voltagepower supply 48 provides the accelerating potential for the electronbeam, which typically ranges from −500 volts to −1000 volts, whilefilament power supply 50, which is referenced to reference power supply48, provides the current for heating filament 28. Bias power supply 52provides a DC control potential to gun grid 30. The isolated outputvoltage of bias power supply 52, referenced to reference power supply48, is controlled by the program input by a voltage from raster/vectorgenerator 43. The beam intensity signals provided by coupler 42 areadded to the voltage produced by bias power supply 52 to modulateelectron beam 26. Coupler power supply 54 provides power to coupler 42,and is also referenced to reference power supply 48.

Exposure control and compensation are used in system 10 to adjust forany electron beam intensity drift over time. Compensation isaccomplished by automatic measurement of the electron beam 26 currentand automatic adjustment of bias power supply 52 voltage by a beamcontrol box 44. When the compensation aspect of raster/vector generatorbeam control box 44 is activated, electron beam 26 is deflected to abeam measurement electrode 56 in electron optics chamber 16, where it iscollected and sent to a beam current amplifier 58. The output voltagefrom beam current amplifier 58 is compared in beam control box 44 with areference voltage from raster/vector generator 43, and the outputvoltage of beam control box 44, which controls bias power supply 52, isautomatically adjusted until the level of electron beam 26 currentcorresponds to the reference voltage. The electron beam 26 current issampled periodically and compared to the reference value fromraster/vector generator 43. The program voltage from beam control box 44to the bias power supply 52 is automatically adjusted to maintain aconsistent level of electron beam 26 current. The reference voltage inbeam control box 44 module can also be controlled by an intensitycontrol 60, allowing different levels of intensity to be set.

The electrons from electron gun assemblies 12 leave anode apertures 38as slowly diverging beams that travel down drift tubes 75 and are thenfocused and positioned onto the printed circuit board 62 by electronoptics assemblies 14. Each of electron optics assemblies 14 consists offive high precision coils 64 for controlling electron beam alignment,focus, position, shape and size; magnetic shielding to eliminateinterference by external magnetic fields; and various circuit modules toprovide signals to control and drive the electron optic coils. Electronoptic coils 64 consist of the following: two alignment yokes 66, 68; ahigh resolution static and dynamic focus coil 70; a high performancedeflection yoke 72; and an astigmatism corrector 74. Since all geometricimage distortions, for example pin cushioning, are correctedelectronically, no geometric correction coils or magnets are required.

Two beam alignment yokes 66, 68 are used to position electron beam 26precisely along the electromagnetic axis of focus coil 70. One beamalignment yoke 66 is mounted directly in front of the anode aperture 38,while the second beam alignment yoke 68 is mounted on the end of a coilhousing assembly 76. By adjusting the X and Y currents in both yokes,electron beam 26 can be precisely aligned with the magnetic field offocus coil 70, with no mechanical adjustment of focus coil 70 required.A single gap static and dynamic focus coil 70 is used in system 10. Itis precisely located and fit into coil housing assembly 76 to minimizethe electrical adjustments required to align electron beam 26 to itsmagnetic axis. No mechanical adjustments of focus coil 70 are required.Electron beam 26 is focused at the center of the test format by the sumof the fields produced by the currents through the static and dynamicwindings of focus coil 70. The current through the static winding is avery stable constant current. The current through the dynamic winding isdynamically adjusted to refocus electron beam 26 as it is deflectedtoward the edges and corners of the test format.

Deflection yoke 72 produces the magnetic fields that precisely positionelectron beam 26 across the entire test format. It uses a ferrite coredesign to provide an anastigmatic field and to eliminate higher orderbeam distortions. A coil winding configuration provides completesymmetry between deflection axes. Consequently, using dynamic focusmodulation, uniform focus over the entire test format can be achieved.An astigmatism corrector 74 is used to compensate for the residualastigmatism of focus coil 70 and the deflection astigmatism produced bydeflection yoke 72. It consists of two coils wound on a common annularcore. In order not to produce a component of deflection, astigmatismcorrector 74 is precisely aligned to the center of the electron opticalaxis. Electron beam astigmatism is corrected at the center of the testformat for astigmatism due to focus coil 70 and then is dynamicallyadjusted to correct the beam shape as it is deflected toward the edgesand corners of the test format.

The entire electron beam 26 path, from electron gun assemblies 12 toprinted circuit board 62 is shielded from the interference of magneticfields emanating from other nearby components by a two-layer magneticshield 78 made of high permeability material.

A number of electron optics circuits supply the drive currents for theelectron optic coils 64 to provide beam alignment, focus, deflection,and astigmatism correction. They also generate signals for the dynamiccorrection of focus, astigmatism, and geometric distortions. A staticfocus control 80 provides constant current to the static winding of thefocus coil 70, while a dynamic focus driver 82 provides drive currentfor the dynamic winding of the focus coil 70. The dynamic focuscorrection signal generated on a geometric correction generator 84 isused to adjust the focus, necessitated by a change in focal length aselectron beam 26 is deflected over its full format. A dynamicastigmatism correction generator 86 generates signals used to correctfor spot distortions as electron beam 26 is deflected away from thecenter of the test format. The signals generated by dynamic astigmatismcorrector 86 are sent to the astigmatism coil driver, of coil drivers88, which use the signals to control astigmatism corrector 74.

Geometric correction generator 84 generates the signals used to correctfor distortions as electron beam 26 is deflected away from the center ofthe test format. The correction signals are summed with the main Xdeflection signal 90 and Y deflection signal 92 (which are the signalspassed to geometric correction generator 84 and dynamic astigmatismcorrector 86) in a deflection amplifier 94. The X deflection signal 90and Y deflection signal 92 are supplied by raster/vector generator 43.The geometric correction generator generates, as a minimum, correctionsignals for X/Y radial linearity, X/Y pincushion, X/Y edge bow, X/Ydifferential linearity, X/Y bow, X skew, Y rotation, X/Y trapezoidal,X/Y edge rotation, X/Y 5th order correction and X/Y 7th ordercorrection. Deflection amplifier 94 receives X deflection signal 90 andY deflection signal 92 as well as signals from geometric correctiongenerator 84 and provides the drive currents for X and Y beampositioning across the test format. When the compensation aspect ofautomatic beam control 44 and electron beam current is sampled forcontrol of the beam intensity level, signals to deflection amplifier 94deflect electron beam 26 to beam measurement electrode 56.

Electron gun assemblies 12, when coupled with electron optics assemblies14, thus provide high speed and precision X, Y and Z (intensity) controlof electron beam 26. Electron gun assemblies 12 generate electron beams26 and vary their intensity according to a control signal fromraster/vector generator 43. Electron optics assemblies 14 shape electronbeams 26 (focus and astigmatism) and deflect the beams (X and Y)according to signals from raster/vector generator 43. Focus, deflection,and astigmatism signals are dynamically adjusted to provide a welldefined and accurately positioned spot over the entire format.

Referring now to FIG. 4, system 10 incorporates a vacuum system 20 toensure for proper operation of electron gun assemblies 12 and formationof a finely focused electron beam 26. Unlike traditional cathode raytube (CRT) devices, which are sealed tubes that will maintain a vacuumindefinitely, system 10 requires that the ‘tube” be opened to theatmosphere to load and unload printed circuit boards 62 to be tested.Alternatively, a load/unload chamber can be added to system 10. Such achamber can house either a single board at a time or multiple boardsthat can then be fed into and out of the test chamber 18. With theload/unload chamber, test chamber 18 and electron optics chambers 16 canremain under the test level vacuums and only the load/unload chambercycles from atmosphere to vacuum.

Vacuum system 20, is a dual, fully automatic, three stage differentiallypumped system that allows rapid access to printed circuit boards 62 andfast pump-down after loading, while maintaining the high vacuum requiredfor long operational life of electron gun assemblies 12. Vacuum system20 consists of five vacuum chambers, vacuum pumps to maintain the propervacuum in each of the chambers, vacuum valves to control the pump-downand venting of the system and provide ease of maintenance and failsafeoperation, vacuum gauges to measure the vacuum in the various sectionsof the system 10, and a vacuum control unit to monitor the output of thegauges and control the sequence of valve operation. Power for vacuumsystem 20 components is controlled by a power control unit. The warm-uptime for vacuum system 20, from a cold start to operational vacuum, isless than one hour. The pump-down time for test chamber 18 is less thanone minute.

Vacuum system 20 is divided into five vacuum chambers. They consist oftwo electron gun chambers 34, two electron optics chambers 16, and onetest chamber 18. It should be noted that only one of each of electrongun chambers 34 and electron optics chambers 16 is shown in FIG. 4 forthe sake of simplicity. Those not shown are essentially arranged in amirror-image to those which are shown. Each type of chamber ismaintained at a different pressure (differentially pumped) duringoperation. Electron gun chambers 34 are operated at very high vacuum atall times to prolong the life of electron gun assemblies 12. Duringtesting of a printed circuit board 62, electron optics chambers 16 aremaintained at a slightly higher pressure (less vacuum) than gun chambers34, and test chamber 18 is at a slightly higher pressure than electronoptics chambers 16 with controlled leakage from one chamber to the next.Typically, electron gun chambers 34 are operated at 10⁻⁷ Torr, electronoptics chambers 16 at 10⁻⁴ Torr and test chamber 18 at 100 millitorr.Alternatively, if a load/unload chamber is part of the system thatchamber is also operated at 100 millitorr. During loading of a printedcircuit board 62, gun chambers 34 can be sealed from the other chambersand electron optics chambers 16 and test chamber 18 can be vented (airlet in) to allow access to test chamber 18.

In order to maintain a different level of vacuum in each of thechambers, five separate pumping systems are used. Because gun chambers34 and electron optics chambers 16 require a higher vacuum than testchamber 18, each use an oil diffusion pump 98, 100 backed by amechanical pump 102, 104. Test chamber 18 (and load/unload chamber, ifused) vacuum is achieved by using a mechanical pump 106 only. Testchamber mechanical pump 106 is also used during the start of vacuumsystem 20 pump-down to evacuate electron optics chambers 16 to a levelwhere the electron optics chambers' diffusion pumps 100 can operate.

The sequencing of the vacuum system from atmospheric pressure tooperational vacuum is accomplished by the use of electromechanicalvacuum valves. Initially, gun ball valves 108 seal gun chambers 34 fromelectron optics chambers 16 and test chamber 18. Gun chambers 34 areunder vacuum from diffusion pumps 98, while test chamber 18 and electronoptics chambers 16 are at atmospheric pressure. High vacuum butterflyvalves 110 seal electron optics chambers 16 from their diffusion pumps100. Roughing ball valves 112 are open, providing a high conductancepath between electron optics chambers 16 and test chamber 18, andsolenoid vent valve 114 is open. When test chamber vacuum is switched ontest chamber mechanical pump 106 starts, vent valve 114 closes and bothtest chamber 18 and electron optics chambers 16 are pumped down togetheruntil a preset pressure is reached in electron optics chambers 16. Atthis time, roughing valves 112 close and high vacuum butterfly valves110 open allowing electron optics chamber diffusion pumps 100 toevacuate electron optics chambers 16 to a lower pressure than testchamber 18. When operational vacuum is achieved in all three chambers,gun valves 108 open, completing the cycle. Gun valves 108 will closeautomatically if any of the chambers rise above operating pressure.

Sequencing from operational vacuum to atmospheric pressure isaccomplished by switching off the test chamber vacuum. Test chambermechanical pump 106 stops, gun valves 108 close, high vacuum butterflyvalves 110 close, roughing valves 112 open, vent valve 114 opens and airis let into test chamber 18 and electron optics chambers 16 to allowaccess to printed circuit board 62. Vent valve 114 provides failsafeoperation in that it is normally closed and must be energized to open.

Two types of vacuum gauges are used in vacuum system 20. Thermocouplegauges 116 are provided for sensing the vacuum at low vacuum levels andcold cathode ionization gauges 118 are provided for sensing the vacuumat high vacuum levels. Thermoelectric baffles 120 are provided betweendiffusion pumps 98 and gun chambers 34 and ambient baffles 122 areprovided between diffusion pumps 100 and electron optics chambers 16 tocondense hot diffusion pump oil and thereby prevent oil backstreaming togun chambers 34 and electron optics chambers 16. Manual vent valves 124located between diffusion pumps 98 and mechanical pumps 102 and betweendiffusion pumps 100 and mechanical pumps 104 can be used to vent thevacuum therebetween. Fans 126 are provided for diffusion pumps 98, 100and thermoelectric baffle 120 to provide cooling, and traps 128 areprovided between diffusion pumps 98 and mechanical pumps 102 to preventfouling of delicate instruments.

Referring now to FIG. 5, the preferable configuration for system 10 isshown. This configuration includes two electron gun assemblies 12 insidetwo gun chambers 34, two electron optics assemblies 14, two electronoptics chambers 161 and a test chamber 18. Two raster/vector generators43 provide the control signals 129 to the two electron guns 12 and thedeflection signals 131 to the two deflection yokes 72. Raster/vectorgenerators 43 are in turn controlled by control software 135 executingon CPU 24. Vacuum system 20 generates the required vacuum environment ofall the chambers in the system. Signals 133 from the two sides ofprinted circuit board 62 are fed into signal processing electronics 194.The processed signals are then sent to CPU 24. Interpretive software 196which resides on CPU 24 is then used to determine whether any shorts oropens are present on printed circuit board 62. System 10 in thisconfiguration can tests both sides of a printed circuit boardsimultaneously.

Referring now to FIGS. 6 and 7, system 10 preferably includes a spotanalyzer 130, which is a diagnostic tool that permits setup of electronoptical systems 16 of system 10. Specifically, spot analyzer 130 isdesigned to permit the following: alignment of focus coils 70,adjustment of static focus, dynamic focus, dynamic astigmatism andgeometry, and measurement of the spot size of electron beams 26. Spotanalyzer 130 includes a target plate 132 having sensors 134 mountedthereon, signal processing electronics 138, and computational software140.

When spot analyzer 130 is to be used, target plate 132 is mounted at thetest plane of system 10 in place of printed circuit board 62. Targetplate 132 contains a plurality of apertures 142 on each face, each ofthe apertures having a polygon shape. These apertures 142 are placed atselected locations over the printed circuit board test format. Targetplate 132 includes a set of sensors 134. These sensors 134 are placedinside the target plate 132 between the two sets of apertures 142. Thesensors 134 are underneath the apertures 142, thereby requiring that theelectron beams 26 pass through apertures 142 before being measures. Eachset of sensors 134 is used to establish and verify geometric correction.In addition, a subset of these sensors 134 is used to establish andverify peak focus and minimum astigmatism over the entire test format.Peak focus is established by observing the slope of the output waveformas electron beam 26 scans across the target edge and maximizing the“steepness” of the slope of this waveform. Similarly, astigmatism isminimized by scanning each target in a ray burst pattern and adjustingastigmatism corrector 74 until the slopes of all the waveforms resultingfrom the ray burst scan are the same.

Beam control signals are provided by the raster/vector generator 43.These signals consist of beam positioning signals and sweep signals. Thebeam positioning signals are X and Y deflection voltages when are fed tothe deflection amplifier 94 of system 10 so that electron beam 26 can bepositioned to any one of the aperture locations. Beam positional signalspermit both coarse and fine positioning adjustment capability. The finepositioning signals can position electron beam 26 to any accuracy of+/−4 microns. The sweep signals are used to scan each of the apertureswith the scan pattern appropriate for the specific operation beingperformed. While the specific scans are a function of the selectedaperture design, the types of scans include raster scans (threedifferent sizes), horizontal line scans, vertical line scans, and rayburst scans. In all cases, the sweep signals are summed with the DCpositioning voltages before being fed into deflection amplifier 94.Where appropriate, trigger pulses for start of scan are also provided.

Sensor input signals 144 being generated by electron beam 26 scanningthe targets are processed as a function of the selected mode ofoperation by signal processing electronics 138. Signal processingincludes amplification, filtering, and analog-to-digital conversion.Computational software 140, which may or may not be executing on CPU 24,is then used in adjusting dynamic focus driver 82, dynamic astigmatismcorrection generator 86, and geometric correction generator 84.Computational software 140 also permits calculation of the linefrequency response (LFR) of the spot, which is a quantitative measure ofthe spot's size and shape.

Referring now to FIGS. 8 and 9, the methodology used in testing aprinted circuit board 62 is based upon the fact that a potential can beestablished on a surface 146 by striking that surface with electrons 148in a particular energy range. That energy range is where the number ofelectrons 150 knocked off the surface (secondary electrons) by thoseelectrons 148 striking the surface are more than the number of electrons148 striking the surface. That energy range may be different fordifferent materials, but generally fall within a predicable envelope. Ifa grid or mesh 152 is placed just above surface 146, and the electrons148 striking the surface are in the range described above, then thatsurface 146 will acquire a potential applied to the grid. For example,if grid 152 is placed at a more positive potential than a referencepotential of the surface (FIG. 8), when electrons 148 strike surface146, essentially all of secondary electrons 150 will be attracted togrid 152, and surface 146 will acquire the positive potential of grid152. For example, if the reference voltage is 0 volts, and the grid isplaced at 20 volts, the surface will acquire the positive 20 voltpotential. However, if grid 152 is placed at a more negative potentialthan a reference potential of the surface (FIG. 9), when electrons 148strike surface 146, essentially all of secondary electrons 150 will berepelled by grid 152 back to surface 146, and surface 146 will acquirethe negative potential of grid 152. For example, if the referencevoltage is 0 volts, and the grid is placed at −20 volts, the surfacewill acquire the negative −20 volt potential. This process is referredto as grid stabilization.

FIG. 10 illustrates a simple printed circuit board 62. Using the gridstabilization approach described above, one or more selected conductortraces 154 on board 62 can be “primed” from a reference voltage to adesired first potential. The time required to prime a trace is afunction of the material of which the trace is made, the length andwidth of the trace, the distance between the trace and other traces,power planes and ground planes, and the amount of beam current in thebeam striking the trace. Typically, however, the time required is on theorder of milliseconds. Referring now to FIG. 11, when one end 156 of atrace 158 is primed on board 62′ that has no shorts or opens, thefollowing will occur: (i) the entire primed trace 158, from the primedend 156 to the unprimed end 160, will acquire the potential applied togrid 152, and (ii) no other traces 154 on board 62′ that are notdeliberately connected to the primed trace 158 will acquire the samepotential. However, as illustrated in FIG. 12, if a particular trace 162of board 62″ has an open 164 in it, that is there is a break somewherealong trace 162, then the end 168 of trace 162 opposite the primed end170 will not acquire the same (i.e., the grid) potential. Similarly, asillustrated in FIG. 13 if two (or more) traces have inadvertently beenelectrical connected (shown at 172), or shorted, then, in addition tothe end 174 of primed trace 176 opposite the primed end 178 acquiringthe grid potential, the non-primed trace 182 and both ends 180 of thenon-primed trace 182 will also acquire the grid potential.

Being able to detect the voltage on the surface of a conductor trace,either at the other end of the trace that has been primed, or onadjacent traces, is essential to determining whether an open or a shortexists. The detection process is accomplished by detecting the flow (orlack of flow) of secondary electrons (or current) in the grid above thecircuit board. Referring now to FIGS. 14 and 15, grid 152 is placed at apotential which is between the potential of unprimed traces (i.e., thereference potential) and primed traces (i.e., the first grid potential).As illustrated in FIG. 14, if the potential on the surface of trace 146being struck by primary beam 148 is more positive than the potential ongrid 152, essentially none of secondary electrons 150 generated byprimary beam 148 will be attracted to grid 152. On the other hand, asillustrated in FIG. 15, if the potential of the surface of the trace 146being struck by primary beam 148 is more negative than grid 152, thenessentially all of secondary electrons 150 generated by primary beam 148will be attracted to grid 152.

Two illustrative examples follow, one using negative potentials with a 0volt reference voltage and the other using positive potentials with a 0volt reference voltage. It should be understood, however, thatcombinations of positive and negative potentials may also be used, andthat the reference voltage may or may not be 0 volts, so long as thesecond grid potential (i.e., the interrogating potential) is between thereference potential and the first grid potential (i.e., the primingpotential).

In the negative potential example, all traces are at a reference voltageof 0 volts. Grid 152 is placed at a first negative potential (e.g., −20volts) and a trace is primed to that potential. Grid 152 is then placedat a second negative potential which is between the potential ofunprimed traces (e.g., 0 volts) and primed traces (e.g., −20 volts). Forexample, grid 152 may be placed at −10 volts. The primary beam 148 isnow used to interrogate various locations on traces on the board. If theinterrogated location on the trace has been primed (i.e., is at −20volts), secondary electrons will be collected by grid 152. If theinterrogated location on the trace has not been primed (i.e., is at 0volts), then secondary electrons will not be collected by grid 152. Thepresence or absence of collected secondary electrons can then be used toindicate shorts or opens. In the positive potential example, all tracesare at a reference voltage of 0 volts. Grid 152 is placed at a firstpositive potential (e.g., 20 volts) and a trace is primed to thatpotential. Grid 152 is then placed at a second positive potential whichis between the potential of unprimed traces (e.g., 0 volts) and primedtraces (e.g., 20 volts). For example, grid 152 may be placed at 10volts. The primary beam 148 is now used to interrogate various locationson traces on the board. If the interrogated location on the trace hasbeen primed (i.e., is at 20 volts), secondary electrons will not becollected by grid 152. If the interrogated location on the trace has notbeen primed (i.e., is at 0 volts), then secondary electrons will becollected by grid 152. The presence or absence of collected secondaryelectrons can then be used to indicate shorts or opens. It should benoted from the above examples that in the negative potential example,the presence of collected secondary electrons indicates that theinterrogated location has been primed, while in the positive potentialexample, the presence of collected secondary electrons indicates thatthe interrogated location has not been primed.

Voltage stabilization grid 152 is an electroformed mesh. The width andspacing of the conductors in this mesh are selected to minimize theinterception of electron beam 26 by the conductors, to maximize thetransmission of electron beam 26 through the mesh, and to insure thatone or more conductors are always in proximity to every conductor traceon printed circuit board 62. In addition, the mesh material should bechemically inert and not readily oxidizable, should not generatemagnetic fields that affect the position of electron beam 26, and shouldhave a high tensile strength. Acceptable materials for grid 152 mayinclude gold, copper, and nickel.

Based upon the principles and description of system 10 componentspresented above, a specific example of the operation of system 10follows. In the case of the manual load system, test chamber 18 is firstbackfilled to atmospheric pressure. During the backfilling operation,electron gun chambers 34 are isolated by gun ball values 108 so thatelectron gun assemblies 12 remain under high vacuum. Test chamber 18door is then opened and printed circuit board 62, or series of boards,to be tested is inserted into a support frame within test chamber 18.Positioned just above the two surfaces of printed circuit board 62 aregrids 152. Once the door to test chamber 18 is closed, a pump-downsequence is initiated that establishes an operating level vacuum in testchamber 18. Because of the relatively low operating vacuum required,this pump-down process takes only between one and two minutes. Once therequired vacuum level has been reached, gun ball valves 108 open andsystem 10 is ready to test printed circuit board 62.

Electron gun assemblies 12 are activated and two electron beams 26,under computer control by raster/vector generator 43, independently scanfiducial marks 184 (FIG. 16) on both sides of printed circuit board 62about to be tested. Secondary electron signals extracted from fiducialmarks 184 are used to either align printed circuit board 62 to thedeflection axes of the electron optics, or more preferably, to align thedeflection axes to printed circuit board 62. The latter approach allowsthe electron optics on each side of printed circuit board to be alignedindependently of the other, which allows for compensation for situationswhere the traces on each side of the board are skewed relative to eachother. Aligning the deflection axes of the electron optics to printedcircuit board 62 can be accomplished by software executing on CPU 24.

Digital data used to manufacture the bare board contain the informationnecessary to position electron beam 26 to the required test points onprinted circuit board 62. This data is loaded into CPU 24, whichcontrols electron beams 26 during the testing operation. During testing,the positioning data is sent by CPU 24 to raster/vector generator 43.Referring now to FIG. 16, the testing sequence consists of firstaddressing one end of a conductor 186, setting the potential on the grid152 above the conductor end 188 to be primed to the desired primingvoltage, turning the electron beam 26 above the conductor end 188 to beprimed on, and priming conductor 186 to that voltage. The voltage ongrid 152 above a desired test end 190 on conductor 186 is then switchedto the desired readout potential and test end 190 on conductor 186 isaddressed, either by the same electron beam 26 performing the priming orby the electron beam 26 on the opposite side of printed circuit board 62if test end 190 is on the opposite side as primed end 188 (as is thecase shown in FIG. 16). The presence or absence of secondary electroncurrent in grid 152 above test end 190 is used by CPU 24 with itsinterpretive software 196 to make a determination as to whether or nottest end 190 has acquired the same potential as primed end 188. If ithas, conductor 186 is continuous. If it has not, an open conditionexists in conductor 186. Subsequently, adjacent conductors 192 areinterrogated by electron beams 26 to determine if they have acquired thesame potential (a short condition).

This testing sequence is repeated for all conductors on both sides ofprinted circuit board 62. During testing, the location and type of anydefects (shorts or opens) detected by CPU 24 with its interpretivesoftware 196 is stored in the CPU for later use. Upon completion of thetesting, test chamber 18 is again backfilled to atmospheric pressure. Asbefore, during the backfilling operation, electron gun chambers 34 areisolated by gun ball values 108 so that electron gun assemblies 12remain under high vacuum. Test chamber 18 door is then opened and theboard, or series of boards, that have been tested is removed.

The fully automatic system differs only in the way printed circuitboards are introduced into and removed from test chamber 18. The fullyautomatic system contains another chamber where a series of boards to betested are initially loaded. This “holding” chamber is connected to thetest chamber by way of a slit valve. With the slit valve closed, thischamber can be opened to atmosphere and a set of boards “stacked”inside. The door to this chamber is then closed and the air insidepumped out. Once the chamber has reached the required vacuum level, theslit valve is opened. A transport mechanism then moves the first board(or set of boards) into the test chamber. When testing of that board iscompleted, it is moved back into the holding chamber and a second boardis moved into the test chamber. This process continues until all of theboards that were placed in the holding chamber have been tested. Theslit valve is then closed, the holding chamber is brought back toatmospheric pressure, the tested boards removed and another set ofboards to be tested are loaded. It should also be understood thatmultiple holding chambers may be provided, so that while one holdingchamber is being used, the other holding chamber(s) may be loaded andunloaded.

The above-described design of system 10 thus allows testing andqualifying of printed circuit boards, or panels of printed circuitboards, up to 18″ by 18″ (324 sq. in.) in size without any movement ofthe boards themselves using an x-y table or the like. This is asignificant improvement over the prior art, with testing of fixedcircuit boards having dimensions greater than 4″ by 4″ (16 sq. in.)being heretofore unheard of. By not requiring mechanical movement of theboards, the time required for testing has been greatly reduced. In thisregard, system 10 is capable of addressing any point on a printedcircuit board up to 18″ by 18″ (324 sq. in.) in size in under 100microseconds. Using two, computer controlled electron beams, all of theconductors on each side of a board, as well as all through boardconductors, are rapidly addressed and interrogated. The computersubsequently analyzes the signals returned from the conductors and adetermination is made as to their integrity. The presence and locationof such defects as an open along a conductor and/or a short betweenadjacent conductors are readily and accurately identified using thissystem. Thus, with the system's electron beam control, correction, andaddressing capability, large area circuit boards can be tested withoutany mechanical movement of the boards. Moreover, the dual electron beamapproach permits testing conductors on both sides of a board“simultaneously,” while also permitting testing of “through board”conductors, i.e. conductors that originate on one side of the board andterminate on the other side.

The present invention, therefore, provides a system for testing printedcircuit boards which tests the networks of printed circuit boardswithout any physical contact therewith, which quickly tests numerouspoints on the boards, which can test large boards or panels of smallboards without requiring movement of the boards, which tests networkspassing through the boards, and which reliably detects the presence ofemitted secondary electrons.

Although the invention has been described with reference to a particulararrangement of parts, features and the like, these are not intended toexhaust all possible arrangements or features, and indeed many othermodifications and variations will be ascertainable to those of skill inthe art.

What is claimed is:
 1. A system for testing for opens in and shortsbetween conductor traces on a surface of a printed circuit board, theconductor traces having a reference potential, said system comprising:an electron gun assembly for generating an electron beam; an electronoptics assembly for directing the electron beam to the conductor traceson the surface of the printed circuit board; a raster/vector generatorfor controlling position and intensity of the electron beam; a gridlocated proximate to and substantially parallel with the surface of theprinted circuit board, said grid capable of collecting secondaryelectrons emitted from the printed circuit board, said grid being placedat a first potential before the electron beam is directed to a firstpoint on a conductor trace to charge the conductor trace to a firstpotential, and being placed at a second potential, the second potentialbeing between the reference potential and the first potential, beforethe electron beam is directed to a second point on a conductor trace tocause emission of secondary electrons; and signal processing electronicsand a CPU for determining whether or not an open or short existsdepending on whether secondary electrons are collected by said grid asthe electron beam is directed to various locations on the conductortraces on the surface of the printed circuit board.
 2. The system ofclaim 1 wherein said electron optics assembly comprises: a focus coilhaving static and dynamic windings to focus the electron beam anywhereacross the surface of the printed circuit board; at least one beamalignment yoke to position the electron beam precisely along themagnetic axis; a deflection yoke to produce magnetic fields thatprecisely position the electron beam anywhere across the surface of theprinted circuit board; and an astigmatism corrector to compensate forany residual astigmatism caused by said focus coil and for anydeflection astigmatism caused by said deflection yoke.
 3. The system ofclaim 2 further comprising a static focus control for controlling thestatic windings of said focus coil.
 4. The system of claim 2 furthercomprising a dynamic focus driver for controlling the dynamic windingsof said focus coil to adjust the focus of the electron beam for changesin focal length as the electron beam is deflected over the surface ofthe printed circuit board.
 5. The system of claim 2 further comprising adynamic astigmatism correction generator for controlling saidastigmatism corrector to correct for spot distortions as the electronbeam is deflected away from a center of the printed circuit board. 6.The system of claim 2 further comprising a geometric correctiongenerator for controlling said deflection yoke to correct for geometricdistortions as the electron beam is deflected away from a center of theprinted circuit board.
 7. The system of claim 1 wherein the referencepotential is substantially 0 volts, wherein the first potentialcomprises a first negative potential, and wherein the second potentialcomprises a second negative potential more positive than the firstnegative potential.
 8. The system of claim 1 wherein the referencepotential is substantially 0 volts, wherein the first potentialcomprises a first positive potential, and wherein the second potentialcomprises a second positive potential more negative than the firstpositive potential.
 9. The system of claim 1 wherein said electron gunassembly is surrounded by a gun chamber, said electron optics assemblyis surrounded by an electron optics chamber, and said printed circuitboard is surrounded by a test chamber, and further comprising a vacuumsystem which creates a vacuum in the test chamber, which creates avacuum in the electron optics chamber greater than the vacuum in thetest chamber, and which creates a vacuum in the gun chamber greater thanthe vacuum in the electron optics chamber.
 10. The system of claim 1further comprising a spot analyzer for adjusting focus, astigmatism andgeometry of the electron beam, and for measuring a spot size of theelectron beam.
 11. The system of claim 10 wherein said spot analyzercomprises: a target plate having a plurality of polygonal apertures on aface thereof; a plurality of sensors positioned inside said target plateunder the plurality of polygonal apertures, said plurality of sensorsgenerating signals indicative of the quality of an electron beamstriking the polygonal aperture over which each sensor is positioned,beam control signals for directing the electron beam to strike theplurality of apertures on the face of said target plate; signalprocessing electronics for amplifying, filtering and performinganalog-to-digital conversion on the signals generated by said pluralityof sensors thereby generating processed signals; and computationalsoftware executing on said CPU for adjusting said electron opticsassembly based on the processed signals.
 12. The system of claim 1wherein the surface of the printed circuit board includes at least twofiducial marks thereon and further comprising software executing on saidCPU for aligning a deflection axes of said electron optics assembly withrespect to the surface of the printed circuit board based upon secondaryelectron signals generated by said at least two fiducial marks whenstruck by the electron beam.
 13. The system of claim 1 wherein thesystem is capable of addressing, with the electron beam, the entiresurface of a printed circuit board having an area greater than 16 squareinches while the printed circuit board is maintained in a stationaryposition with respect to the electron gun assembly.
 14. The system ofclaim 1 wherein the system is capable of addressing, with the electronbeam, the entire surface of a printed circuit board having an areagreater than 36 square inches while the printed circuit board ismaintained in a stationary position with respect to the electron gunassembly.
 15. The system of claim 1 wherein the system is capable ofaddressing, with the electron beam, any point on the surface of aprinted circuit board having an area greater than 16 square inches inless than 100 microseconds.
 16. The system of claim 1 wherein the systemis capable of addressing, with the electron beam, any point on thesurface of a printed circuit board having an area greater than 36 squareinches in less than 100 microseconds.
 17. A system for testing for opensin and shorts between conductor traces on two surfaces of a printedcircuit board, the conductor traces having a reference potential, saidsystem comprising: two electron gun assemblies for generating twoelectron beams; two electron optics assemblies, one of said electronoptics assemblies for directing one of said electron beams to theconductor traces on one of the surfaces of the printed circuit board,and the other of said electron optics assemblies for directing the otherof said electron beams to the conductor traces on the other of thesurfaces of the printed circuit board; a raster/vector generator forcontrolling position and intensity of the electron beams; two grids,each of said grids located proximate to and substantially parallel withone of the surfaces of the printed circuit board, said grids capable ofcollecting secondary electrons emitted from the printed circuit board,at least one of said grids being placed at a first potential before oneof the electron beams is directed to a first point on a conductor traceto charge the conductor trace to a first potential, and at least one ofsaid grids being placed at a second potential, the second potentialbeing between the reference potential and the first potential, beforeone of the electron beams is directed to a second point on a conductortrace to cause emission of secondary electrons; and signal processingelectronics and a CPU for determining whether or not an open or shortexists depending on whether secondary electrons are collected by saidgrids as the electron beams are directed to various locations on theconductor traces on the surfaces of the printed circuit board.
 18. Thesystem of claim 17 wherein each of said electron optics assembliescomprises: a focus coil having static and dynamic windings to focus eachelectron beam anywhere across the surface of the printed circuit board;at least one beam alignment yoke to position each electron beamprecisely along the magnetic axis; a deflection yoke to produce magneticfields to precisely position each electron beam anywhere across thesurfaces of the printed circuit board; and an astigmatism corrector tocompensate for any residual astigmatism caused by said focus coil andfor any deflection astigmatism caused by said deflection yoke.
 19. Thesystem of claim 18 further comprising a static focus control forcontrolling the static windings of said focus coils.
 20. The system ofclaim 18 further comprising a dynamic focus driver for controlling thedynamic windings of said focus coils to adjust the focus of the electronbeams for changes in focal length as the electron beams are deflectedover the surfaces of the printed circuit board.
 21. The system of claim18 further comprising a dynamic astigmatism correction generator forcontrolling said astigmatism correctors to correct for spot distortionsas the electron beams are deflected away from a center of the printedcircuit board.
 22. The system of claim 18 further comprising a geometriccorrection generator for controlling said deflection yokes to correctfor geometric distortions as the electron beams are deflected away froma center of the printed circuit board.
 23. The system of claim 17wherein the reference potential is substantially 0 volts, wherein thefirst potential comprises a first negative potential, and wherein thesecond potential comprises a second negative potential more positivethan the first negative potential.
 24. The system of claim 17 whereinthe reference potential is substantially 0 volts, wherein the firstpotential comprises a first positive potential, and wherein the secondpotential comprises a second positive potential more negative than thefirst positive potential.
 25. The system of claim 17 wherein saidelectron gun assemblies are surrounded by gun chambers, said electronoptics assemblies are surrounded by electron optics chambers, and saidprinted circuit board is surrounded by a test chamber, and furthercomprising a vacuum system which creates a vacuum in the test chamber,which creates vacuums in the electron optics chambers greater than thevacuum in the test chamber, and which creates vacuums in the gunchambers greater than the vacuums in the electron optics chambers. 26.The system of claim 17 further comprising a spot analyzer for adjustingfocus, astigmatism and geometry of the electron beams, and for measuringspot sizes of the electron beams.
 27. The system of claim 26 whereinsaid spot analyzer comprises: a target plate having two opposing facesand a plurality of polygonal apertures on each face of said targetplate; a plurality of sensors positioned inside said target place underthe plurality of polygonal apertures, said plurality of sensorsgenerating first signals indicative of the quality of a first electronbeam striking the apertures on one face of the target plate and secondsignals indicative of the quality of a second electron beam striking theapertures on the other face of the target plate; beam control signalsfor directing the electron beams to strike the plurality of apertures oneach face of said target plate; signal processing electronics foramplifying, filtering and performing analog-to-digital conversion of thefirst signals and the second signals thereby generating first processedsignals and second processed signals; and computational softwareexecuting on a CPU for adjusting one of the electron optics assembliesbased on the first processed signals and for adjusting the other one ofthe electron optics assemblies based on the second processed signals.28. The system of claim 17 wherein each surface of the printed circuitboard includes at least two fiducial marks thereon and furthercomprising software executing on said CPU for aligning the deflectionaxes of one of the electron optics assemblies with respect to a firstsurface of the printed circuit board based upon secondary electronsignals generated by said at least two fiducial marks on the firstsurface of the printed circuit board when struck by one of the electronbeams, and for aligning the deflection axes of the other one of theelectron optics assemblies with respect to a second surface of theprinted circuit board based upon secondary electron signals generated bysaid at least two fiducial marks on the second surface of the printedcircuit board when struck by the other one of the electron beams. 29.The system of claim 17 wherein the system is capable of addressing, withthe electron beams, the entire surfaces of a printed circuit boardhaving an area greater than 16 square inches while the printed circuitboard is maintained in a stationary position with respect to theelectron gun assemblies.
 30. The system of claim 17 wherein the systemis capable of addressing, with the electron beams, the entire surfacesof a printed circuit board having an area greater than 36 square incheswhile the printed circuit board is maintained in a stationary positionwith respect to the electron gun assemblies.
 31. The system of claim 17wherein the system is capable of addressing, with the electron beams,any point on the surfaces of a printed circuit board having an areagreater than 16 square inches in less than 100 microseconds.
 32. Thesystem of claim 17 wherein the system is capable of addressing, with theelectron beams, any point on the surfaces of a printed circuit boardhaving an area greater than 36 square inches in less than 100microseconds.
 33. A method for the contactless testing for opens in aconductor trace passing through a printed circuit board from a firstsurface to a second surface, the conductor trace having a referencepotential, said method comprising the steps of placing a first gridlocated proximate to and substantially parallel with the first surfaceof the printed circuit board to a first potential; directing a firstelectron beam to a first point on the conductor trace on the firstsurface to charge the conductor trace to substantially the firstpotential; placing a second grid located proximate to and substantiallyparallel with the second surface of the printed circuit board to asecond potential, the second potential being between the referencepotential and the first potential; directing a second electron beam to asecond point on the conductor trace on the second surface to causeemission of secondary electrons; and determining whether an open existsdepending on whether secondary electrons are collected by the secondgrid.
 34. The method of claim 33 wherein the reference potential issubstantially 0 volts, wherein said placing a first grid step comprisesthe step of placing a first grid located proximate to and substantiallyparallel with the first surface of the printed circuit board to a firstnegative potential, and wherein said placing a second grid stepcomprises the step of placing a second grid located proximate to andsubstantially parallel with the second surface of the printed circuitboard to a second negative potential, the second negative potentialbeing more positive than the first negative potential.
 35. The method ofclaim 34 wherein said determining step comprises the steps of:determining that no open exists between the first point and the secondpoint if secondary electrons are collected by the second grid; and,determining that an open exists between the first point and the secondpoint if secondary electrons are not collected by the second grid. 36.The method of claim 33 wherein the reference potential is substantially0 volts, wherein said placing a first grid step comprises the step ofplacing a first grid located proximate to and substantially parallelwith the first surface of the printed circuit board to a first positivepotential, and wherein said placing a second grid step comprises thestep of placing a second grid located proximate to and substantiallyparallel with the second surface of the printed circuit board to asecond positive potential, the second positive potential being morenegative than the first positive potential.
 37. The method of claim 36wherein said determining step comprises the steps of: determining thatan open exists between the first point and the second point if secondaryelectrons are collected by the second grid; and, determining that noopen exists between the first point and the second point if secondaryelectrons are not collected by the second grid.
 38. The method of claim33 wherein each of said directing steps comprise: focusing the electronbeam anywhere across the surface of the printed circuit board with afocus coil having static and dynamic windings; positioning the electronbeam precisely along the magnetic axis with at least one beam alignmentyoke; producing magnetic fields to precisely position the electron beamon the surface of the printed circuit board with a deflection yoke; andcompensating for any residual astigmatism caused by the focus coil andfor any deflection astigmatism caused by the deflection yoke with anastigmatism corrector.
 39. The method of claim 38 further comprising thestep of controlling the static windings of the focus coil with a staticfocus control.
 40. The method of claim 38 further comprising the step ofcontrolling the dynamic windings of the focus coil to adjust the focusof the electron beam for changes in focal length as the electron beam isdeflected over the surface of the printed circuit board with a dynamicfocus driver.
 41. The method of claim 38 further comprising the step ofcontrolling the astigmatism corrector to correct for spot distortions asthe electron beam is deflected away from a center of the printed circuitboard with a dynamic astigmatism correction generator.
 42. The method ofclaim 30 further comprising the step of controlling the deflection yoketo correct for geometric distortions as the electron beam is deflectedaway from a center of the printed circuit board with a geometriccorrection generator.
 43. The method of claim 33 further comprising thestep of controlling position and intensity of the electron beam with araster/vector generator.
 44. The method of claim 33 wherein the electronbeams are generated by electron gun assemblies surrounded by gunchambers, wherein the electron beams are directed by electron opticsassemblies surrounded by electron optics chambers, wherein the printedcircuit board is surrounded by a test chamber, and further comprisingthe steps of: creating a vacuum in the test chamber; creating vacuums inthe electron optics chambers greater than the vacuum in the testchamber; and creating vacuums in the gun chambers greater than thevacuums in the electron optics chambers.
 45. The method of claim 33further comprising the step of adjusting focus, astigmatism and geometryof the electron beams, and measuring a spot size of the electron beams,with a spot analyzer.
 46. The method of claim 45 wherein said adjustingand measuring step comprises: providing a target plate having aplurality of polygonal apertures faces thereof; providing a plurality ofsensors positioned inside the target plate under the plurality ofpolygonal apertures; directing the electron beams to strike theplurality of apertures; generating signals indicative of the quality ofthe electron beams striking the polygonal apertures; amplifying,filtering and performing analog-to-digital conversion on the signals togenerate processed signals; and adjusting electron optics assembliesused to direct the electron beams based on the processed signals. 47.The method of claim 33 wherein each surface of the printed circuit boardincludes at least two fiducial marks thereon and further comprising thestep of aligning a deflection axis of each of two electron opticsassemblies, used to direct the electron beams, with respect to thesurfaces of the printed circuit board based upon secondary electronsignals generated by the at least two fiducial marks.
 48. The method ofclaim 33 further comprising the step of holding the printed circuitboard in a stationary position and wherein the system is capable ofaddressing, with the electron beams, the entire surfaces of a printedcircuit board having an area greater than 16 square inches.
 49. Thesystem of claim 33 further comprising the step of holding the printedcircuit board in a stationary position and wherein the system is capableof addressing, with the electron beams, the entire surfaces of a printedcircuit board having an area greater than 36 square inches.
 50. Thesystem of claim 33 wherein the printed circuit board has an area greaterthan 16 square inches, and wherein said directing steps require lessthan 100 microseconds.
 51. The system of claim 33 wherein the printedcircuit board has an area greater than 36 square inches, and whereinsaid directing steps require less than 100 microseconds.
 52. A methodfor the contactless testing for shorts between conductor traces passingthrough a printed circuit board from a first surface to a secondsurface, the conductor traces having a reference potential, said methodcomprising the steps of: placing a first grid located proximate to andsubstantially parallel with the first surface of the printed circuitboard to a first potential; directing a first electron beam to a pointon a first conductor trace on the first surface to charge the firstconductor trace to substantially the first potential; placing a secondgrid located proximate to and substantially parallel with the secondsurface of the printed circuit board to a second potential, the secondpotential being between the reference potential and the first potential;directing a second electron beam to a point on a second conductor traceon the second surface to cause emission of secondary electrons; anddetermining whether a short exists depending on whether secondaryelectrons are collected by the second grid.
 53. The method of claim 52wherein the reference potential is substantially 0 volts, wherein saidplacing a first grid step comprises the step of placing a first gridlocated proximate to and substantially parallel with the first surfaceof the printed circuit board to a first negative potential, and whereinsaid placing a second grid step comprises the step of placing a secondgrid located proximate to and substantially parallel with the secondsurface of the printed circuit board to a second negative potential, thesecond negative potential being more positive than the first negativepotential.
 54. The method of claim 53 wherein said determining stepcomprises the steps of: determining that a short exists between thefirst conductor trace and the second conductor trace if secondaryelectrons are collected by the second grid; and, determining that noshort exists between the first conductor trace and the second conductortrace if secondary electrons are not collected by the second grid. 55.The method of claim 52 wherein the reference potential is substantially0 volts, wherein said placing a first grid step comprises the step ofplacing a first grid located proximate to and substantially parallelwith the first surface of the printed circuit board to a first positivepotential, and wherein said placing a second grid step comprises thestep of placing a second grid located proximate to and substantiallyparallel with the second surface of the printed circuit board to asecond positive potential, the second positive potential being morenegative than the first positive potential.
 56. The method of claim 55wherein said determining step comprises the steps of: determining thatno short exists between the first conductor trace and the secondconductor trace if secondary electrons are collected by the second grid;and, determining that a short exists between the first conductor traceand the second conductor trace if secondary electrons are not collectedby the second grid.
 57. The method of claim 52 wherein each of saiddirecting steps comprise: focusing the electron beam anywhere across thesurface of the printed circuit board with a focus coil having static anddynamic windings; positioning the electron beam precisely along themagnetic axis with at least one beam alignment yoke; producing magneticfields to precisely position the electron beam on the surface of theprinted circuit board with a deflection yoke; and compensating for anyresidual astigmatism caused by the focus coil and for any deflectionastigmatism caused by the deflection yoke with an astigmatism corrector.58. The method of claim 57 further comprising the step of controllingthe static windings of the focus coil with a static focus control. 59.The method of claim 57 further comprising the step of controlling thedynamic windings of the focus coil to adjust the focus of the electronbeam for changes in focal length as the electron beam is deflected overthe surface of the printed circuit board with a dynamic focus driver.60. The method of claim 57 further comprising the step of controllingthe astigmatism corrector to correct for spot distortions as theelectron beam is deflected away from a center of the printed circuitboard with a dynamic astigmatism correction generator.
 61. The method ofclaim 57 further comprising the step of controlling the deflection yoketo correct for geometric distortions as the electron beam is deflectedaway from a center of the printed circuit board with a geometriccorrection generator.
 62. The method of claim 52 further comprising thestep of controlling position and intensity of the electron beam with araster/vector generator.
 63. The method of claim 52 wherein the electronbeams are generated by electron gun assemblies surrounded by gunchambers, wherein the electron beams are directed by electron opticsassemblies surrounded by electron optics chambers, wherein the printedcircuit board is surrounded by a test chamber, and further comprisingthe steps of: creating a vacuum in the test chamber; creating vacuums inthe electron optics chambers greater than the vacuum in the testchamber; and creating vacuums in the gun chambers greater than thevacuums in the electron optics chambers.
 64. The method of claim 52further comprising the step of adjusting focus, astigmatism and geometryof the electron beams, and measuring a spot size of the electron beams,with a spot analyzer.
 65. The method of claim 64 wherein said adjustingand measuring step comprises: providing a target plate having aplurality of polygonal apertures faces thereof; providing a plurality ofsensors positioned inside the target plate under the plurality ofpolygonal apertures; directing the electron beams to strike theplurality of apertures; generating signals indicative of the quality ofthe electron beams striking the polygonal apertures; amplifying,filtering and performing analog-to-digital conversion on the signals togenerate processed signals; and adjusting electron optics assembliesused to direct the electron beams based on the processed signals. 66.The method of claim 52 wherein each surface of the printed circuit boardincludes at least two fiducial marks thereon and further comprising thestep of aligning a deflection axis of each of two electron opticsassemblies, used to direct the electron beams, with respect to thesurfaces of the printed circuit board based upon secondary electronsignals generated by the at least two fiducial marks.
 67. The method ofclaim 52 further comprising the step of holding the printed circuitboard in a stationary position and wherein the system is capable ofaddressing, with the electron beams, the entire surfaces of a printedcircuit board having an area greater than 16 square inches.
 68. Thesystem of claim 52 further comprising the step of holding the printedcircuit board in a stationary position and wherein the system is capableof addressing, with the electron beams, the entire surfaces of a printedcircuit board having an area greater than 36 square inches.
 69. Thesystem of claim 52 wherein the printed circuit board has an area greaterthan 16 square inches, and wherein said directing steps require lessthan 100 microseconds.
 70. The system of claim 52 wherein the printedcircuit board has an area greater than 36 square inches, and whereinsaid directing steps require less than 100 microseconds.